Vertical Deflection Problem: Test +25VDC at TP31 or IC103 pin 14 If missing R167 may be open. Check for broken V hold pot on remote board. Sub IC103, IC104. Test R177. Replace C144, C142, C147.
one of my Polos used to have this problem, until I eliminated the remote cabling and just plugged the board directly into the chassis. made my adjustments and haven't touched it since.
it used to do the same thing you speak of, try tinkering with the V-size pot (you can try wiping it with the machine off, can't hurt) and see if you get a full picture again.
otherwise, I don't know. I think I got some cold solder on mine, but I think you can understand full well why I don't mess with it, cause it's a Polo, and in my experiences with them, if they work, leave them alone.